1. Field of the Invention
The present invention relates to a wavelength-selecting optical element to be incorporated into an optical display, such as a liquid crystal display, and more particularly, to a wavelength-selecting optical element adapted to selectively reflect or transmit a circularly polarized light component of a selected wavelength rotating in one direction, and an optical display provided with the same.
2. Description of the Related Art
A wavelength-selecting optical element provided with a cholesteric layer has been employed as a color filter in conventional liquid crystal displays and the like. The term “liquid crystal layer” used herein means a layer having the properties of liquid crystal in the optical sense, and includes not only a layer of liquid crystal phase having flowability but also a layer of solid phase obtained by solidifying liquid crystal phase while retaining the alignment of molecules characteristic of the liquid crystal phase.
The cholesteric layer has a rotatory light-selecting characteristic (polarized light separating characteristic) capable of separating a circularly polarized light component of one direction and that of the opposite direction from each other on the basis of physical molecular arrangement (planar arrangement). Light incident on such a cholesteric layer along the helical axis of a planar arrangement is separated into a right-handed circularly polarized light component and a left-handed circularly polarized light component. Either the right-handed or the left-handed circularly polarized light component is reflected and the other light component is transmitted. This phenomenon is generally known as circular dichroism. The rotating direction of the circularly polarized light component of the incident light is selected properly to reflect or transmit a circularly polarized light component having a rotating direction corresponding to that of the helical axis of the cholesteric layer. A maximum optical rotatory dispersion of an outgoing light occurs at a wavelength λ0 expressed by Expression (1):λ0=nav·p,  (1)where p is chiral pitch (helical pitch) and nav is average refractive index in a plane perpendicular to the helical axis.
Bandwidth Δλ of wavelengths of the outgoing light is expressed by Expression (2):Δλ=Δn·p,  (2)where Δn is index of birefringence and p is chiral pitch.
Thus, the cholesteric layer reflects one of a right-handed of a left-handed circularly polarized light component in the wavelength range of a bandwidth Δλ having a center wavelength λ0 and transmits the other circularly polarized light component and light waves of wavelengths outside the aforesaid wavelength range.
When the helical axis of the cholesteric layer of this known wavelength-selecting optical element is, for example, parallel to a normal to a substrate, the apparent pitch of the cholesteric layer increases when the cholesteric layer is viewed from a direction oblique to the normal to the substrate. Therefore, the wavelength of the light wave reflected or transmitted by the cholesteric layer shifts toward the side of shorter wavelengths.
Expression (1) is modified by taking a viewing angle θ (angle of a viewing direction to the normal to the surface of the substrate) into consideration and obtained is Expression (3):λ0=nav·p·cos θ.  (3)
It is known from Expression (3) that the wavelength λ0 decreases with the increase of the viewing angle θ. Therefore, the hues of colors of an image displayed by an optical display provided with this wavelength-selecting optical element vary for color shift according to the viewing direction.